1. Field of the Invention
The present invention relates to a technique for improving the strength of a battery, such as a polymer battery, having a film-type casing.
2. Description of the Related Art
In recent years, with the widespread proliferation of compact electronic devices, such as mobile phones, portable audio devices, digital cameras, and personal digital assistants (PDAs), there is a rapidly growing demand for a battery that is thin, lightweight, and yet has high energy density. To meet this demand, there has been provided a battery, such as a lithium ion polymer electrolyte battery, having a film-type casing of a slim rectangular parallelepiped shape (the term xe2x80x9cslimxe2x80x9d used herein refers to the order of a few millimeters). To be more specific, it is a new type of battery having an electrode body formed in a slim rectangular parallelepiped shape, and housed in a casing that is constituted of a flexible, extremely thin film folded together. Such a battery can be extremely slim and lightweight while being comparatively large in energy density. For this reason, expectations run high for such a battery as a promising power supply for the sort of compact electronic devices mentioned above.
Generally, because the casing is flexible, a battery having a film-type casing is easily deformed (especially, bent on a side or concaved by depression) when dropped for instance. When the film-type casing is deformed, the electrode body housed therein is often deformed as well, which results in the possibility of an electric short.
One solution to address this problem is to attach a plastic-molded reinforcing plate at the bottom of the battery so as to absorb impact of a drop. This reinforcing plate, however, causes a reduced energy density because of the space that the reinforcing plate occupies within the battery.
As described above, there still are problems to be solved regarding a battery having a film-type casing.
An object of the present invention is to provide a battery having a film-type casing which is excellent in energy density as well as in strength.
The above object of the present invention is achieved by a battery housed in a casing that is constituted of a film folded together, the battery comprising: an electrode body formed of at least one positive electrode plate and at least one negative electrode plate, and a separator sandwiched between the electrode plates, and having at least one rectangular-shaped outer surface; and a reinforcing sheet attached to the electrode body so as to cover an area of the rectangular-shaped outer surface corresponding to at least one corner of the positive electrode plate.
Here, the present invention is especially effective when the negative electrode plate and the positive electrode plate are band-shaped, the negative electrode plate is wider than the positive electrode plate, and the electrode body is a spiral electrode body formed by (1) winding the positive electrode plate and the negative electrode plate together with the separator sandwiched therebetween, and (2) compressing the wound electrode plates from a circumferential direction so as to form the rectangular-shaped outer surface.
Further, the reinforcing sheet may also function as a wind-fastener for securing a winding-end of the electrode plate forming an outermost layer of the electrode body.
Still further, the present invention is especially effective when the positive electrode plate forms the outer surface of the electrode body.